The international astronomers offer evidence of clouds circling the supermassive black holes by analyzing the records offered by RXTE during its 16 years of mission.

Ever since it was launched in 1995 aboard Delta II rocket, NASA's RXTE has made incredible findings. This low orbit satellite was equipped with several instruments that helped in measuring the variations in X-ray sources. The various X-ray sources include active galactic nuclei, which are bright shining objects that are powered by the supermassive black holes as they collect together and condense large volume of dust and gas. The aging satellite was later decommissioned in January 2012.

Carefully checking through records of 55 different active galatic nuclei, the team discovered a few cases where the X-ray signals were faint for a period of time that varied from a few hours to years, with wide energy variations from 2,000 to 250,000 electron volts. This occurred mostly when a dense gas moved between the satellite and the source. This finding reveals a 3-fold increase in the number of cloud events that were indentified earlier in the 16-year archive.

The observation was made by Alex Markowitz, an astrophysicist at the University of California, San Diego and the Karl Remeis Observatory in Bamberg, Germany and colleagues.

In the centre of most of the massive galaxies, including our own galaxy The Milky Way, there exists a supermassive black hole that is millions times the weight of the Sun's mass. When the cloud of gas accelerates toward the black hole, it forms into an accretion disk and gets compressed and heated that eventually leads to the emission of X-rays. In a few galaxies, the centres produce a powerful emission that surpasses the energy output of the sun by nearly billions of times. These are referred to as active galactic nuclei or AGN.

"One of the great unanswered questions about AGN is how gas thousands of light-years away funnels into the hot accretion disk that feeds the supermassive black hole," said Alex Markowitz, an astrophysicist at the University of California, San Diego and the Karl Remeis Observatory in Bamberg, Germany. "Understanding the size, shape and number of clouds far from the black hole will give us a better idea of how this transport mechanism operates."

This latest study is the first statistical survey of the environments surrounding the supermassive black holes. Apart from this, the study is the longest AGN monitoring study performed in X-rays.

"Because RXTE performed sustained observations of many of these AGN, our research is sensitive to a wide range of cloud events, from those as brief as five hours to as long as 16 years," said co-author Robert Nikutta, a theorist at Andrés Bello University in Santiago, Chile.

The study scientists explain the range of properties of occulting clouds that again vary in size and shape and orbit a few light weeks to a few light years from the black hole. A cloud spotted in a spiral galaxy, located 143 million light year away in the direction of the constellation Centaurus (NGC 3783), appeared in the midst of being ripped apart by a tidal force.

"In 2008, the AGN dimmed twice over a period of 11 days and did not reach its typical X-ray brightness within that period," said co-author Mirko Krumpe of the European Southern Observatory in Garching, Germany. "This could be caused by an elongated, filamentary cloud, perhaps one that is in the process of being torn apart by the black hole."